1. Field of the Invention
The present invention relates to a method and apparatus for optical scanning, and more particularly to a method and apparatus for optical scanning capable of performing a high speed and high pixel density scanning.
2. Discussion of the Background
In an image forming apparatus such as a digital copying machine, a laser printer, or the like, a background optical scanning apparatus for generating a scanning laser light beam is generally used. The background optical scanning apparatus has a rotary polygon mirror configured to be rotated to sequentially deflect a laser light beam by reflection and to cause the laser light beam to scan a charged imaging surface of a photosensitive member. By continuing the scanning, an electrostatic latent image is reproduced on the imaging surface.
A recent demand for a high image density and a high speed with respect to the digital copying machines and the laser printers has been addressed mostly by increasing a rotational seed of the rotary polygon mirror and/or by changing to a multi-beam system by increasing a number of light sources of the background optical scanning apparatus.
When the speed of the rotary polygon mirror is indiscriminately increased, however, it may cause various drawbacks such as a generation of noise and vibration, an increase of electric consumption, etc. An indiscriminate change to a multi-beam system may also cause various drawbacks such as an increase of cost for the light source, variations in synchronization of the multiple beams, and so on. In particular, variations in pitch among the multiple beams may cause displacements of colors in an image.
Therefore, the present inventors have recognized that the background optical scanning apparatus has a need to be improved in the rotational speed of the rotary polygon mirror and the change to the multi-beams system with keeping an appropriate balance therebetween.
Further, the background optical scanning apparatus having a beam spot of from approximately 60 xcexcm to approximately 70 xcexcm causes the following problems with increasing image density. For example, at a high density of 1200 dpi or more, a pixel pitch is reduced to 21 xcexcm, which is smaller than the above-mentioned beam spot, causing an excessive overlapping between an adjacent two image dots. As a result, a halftone reproducibility in a gray-scale image such as a photo image is degraded.
For another example, with a larger beam spot, an energy density in the beam power running on the photosensitive member is decreased and consequently a potential of the electrostatic latent image formed on the surface of the photosensitive member becomes unstable. This may cause a difficulty in developing a single dot image with toner in the developing process, which is apt to be apparent particularly when the developing conditions are changed over time and may cause a degradation of an image granularity.
These problems with increasing image density may be effectively avoided by reducing the beam spot to 50 xcexcm or smaller.
To reduce the beam spot, however, a size of a laser light beam entering a scanning lens needs to be larger to increase a numerical aperture (NA) indicating an opening efficiency. Accordingly, a width of the laser light beam entering a deflective reflection surface of a rotary polygon mirror is increased and, as a result, a radius of an inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror needs to be increased. However, when a radius of an inscribed circle is made larger, an air resistance is increased and various problems, such as a generation of a wind noise, vibration, jitter, etc., an increase of electric consumption, and so on, may occur particularly when the rotary polygon mirror is rotated at a high speed.
Therefore, a revolution number of the rotary polygon mirror is needed to be suitably determined to avoid the above-described problems when a radius of an inscribed circle is made larger.
The deflective scanning method by the optical scanning system can be grouped into an underfilled optical system and an overfilled optical system. The underfilled optical system, which is most-widely used, is provided with an aperture between a light source and a rotary polygon mirror serving as a light deflection device, in which a deflective reflection surface of the rotary polygon mirror is wider than a size of the light beam exiting from the aperture. That is, the underfilled optical system satisfies a relationship of:
xcex6 less than xcex7,
wherein xcex6 represents a size of the light beam and xcex7 represents a width of a deflective reflection surface of the rotary polygon mirror.
The underfilled optical system has an advantage in making a smaller beam spot while being resistant to a generation of bend in the scanning line. The underfilled optical system, however, has a drawback of increasing a radius of an inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror. That is, the inscribed circle radius, which already has a relatively large size since it is made larger than the entry beam spot size, is further increased when a number of the deflective reflection surfaces of the rotary polygon mirror is increased. Therefore, there is a limit in increasing the speed of the rotary polygon mirror due to the above-mentioned wind noise.
On the other hand, the overfilled optical system has slowly been used as a system with the advantage of increasing the speed of the rotary polygon mirror. This is because the overfilled optical system uses the deflective reflection surfaces as the actual aperture and has the laser light beam spot that enters the deflective reflection surface larger than the size of the deflective reflection surface.
In the overfilled optical system, a reduction of the inscribed circle radius with respect to the deflective reflection surfaces of the rotary polygon mirror and an increase of the number of the deflective reflection surfaces of the rotary polygon mirror may be achieved in a relatively easy manner. Therefore, the overfilled optical system has an advantage in increasing the speed of the rotary polygon mirror.
However, the overfilled optical system generally has the following drawbacks relative to the underfilled optical system. As one example of a drawback, the laser light beam needs to have a tilt angle in the sub-scanning direction to enter the deflective reflection surface of the rotary polygon mirror with a beam axis towards the rotational center of the rotary polygon mirror. Because of this, a tangential coma may be generated. Therefore, the overfilled optical system has a difficulty in reducing the diameter of the laser light beam spot and is consequently apt to cause bend in the scanning line.
For another example of a drawback, an aperture diameter in the main scanning direction and a light amount are varied by the light deflection in the overfilled optical system. Due to this, an angle of field may not be increased. As a result, a light path becomes longer, which makes the optical scanning apparatus large.
As described above, the underfilled and overfilled optical systems have advantages and the disadvantages opposed to each other. In each system, it is possible to realize an optical system having merits in a manufacturing cost, an electric consumption, a noise, and a beam spot reduction by optimizing the revolution number of the rotary polygon mirror and the beam number generated by the light source.
In view of the foregoing, it is an object of the present invention to provide a novel optical scanning apparatus that performs a high speed and high pixel density scanning.
Another object of the present invention is to provide a novel optical scanning method that performs a high speed and high pixel density scanning.
Another object of the present invention is to provide a novel image forming apparatus that performs a high speed and high pixel density scanning.
To achieve the above-mentioned and other objects, in one example, a novel optical scanning apparatus using an underfilled optical system includes a predetermined number M of light sources, a first optical scanning lens system, a second optical scanning lens system, a rotary polygon mirror, and a third optical scanning lens system. The predetermined number M of light sources emit a laser light beam. The first optical scanning lens system is configured to perform a coupling process relative to the laser light beam emitted from the predetermined number M of light sources. The second optical scanning lens system is configured to gather light of the laser light beam from the first optical scanning lens system in an approximately linear state extended in a main scanning direction. The rotary polygon mirror has a predetermined number N of deflective reflection surfaces and is configured to receive and deflect the laser light beam gathered in the approximately linear state. The third optical scanning lens system is configured to gather the deflected laser light beam from the rotary polygon mirror to form a beam spot on an imaging surface. In this optical scanning apparatus, the predetermined number M satisfies a condition:
3xc3x97Rp/Rmaxxe2x89xa7Mxe2x89xa7Rp/Rmax,
wherein Rp and Rmax are defined as:
Rpxe2x89xa1(Dpi/25.4)xc3x97(260xc3x97Ppm)/N,
Rmaxxe2x89xa1(5.4xc3x97106)xc3x97√{N1.6/(A4xc3x97t)}, respectively,
wherein M is greater than two, Rp is a revolution number (rpm) of the rotary polygon mirror in a single beam mode, Rmax is a maximum revolution number (rpm) of the rotary polygon mirror, Dpi is a pixel density (dpi) in a sub-scanning direction, Ppm is a print speed (ppm) expressed in a number of A4-sized print pages in a landscape orientation, N is a number of the deflective reflection surfaces of the rotary polygon mirror, A is a radius (mm) of an inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror, and t is a thickness (mm) of each deflective reflection surface of the rotary polygon mirror.
A diameter xcfx89m of the beam spot in the main scanning direction, formed on the imaging surface by the third optical scanning lens system, may be a 1/e2 diameter and is equal to or smaller than 50 xcexcm, and the radius A of the inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror may satisfy a condition:
Axe2x89xa7(0.76xc3x97fmxc3x97xcex/xcfx89m)/tan {(180xc2x0/N)xe2x88x92(xcex8/2)},
wherein xcex (mm) is a central wave length of the light source, fm (mm) is a focal distance of the third optical scanning lens system in the main scanning direction, and xcex8 is a half field angle of the third optical scanning lens system, including a synchronizing laser light beam.
In another example, a novel optical scanning apparatus using an overfilled optical system includes a predetermined number M of light sources, a first optical scanning lens system, a second optical scanning lens system, a rotary polygon mirror, and a third scanning lens system. The predetermined number M of light sources emit a laser light beam. The first optical scanning lens system is configured to perform a coupling process relative to the laser light beam emitted from the predetermined number M of light sources. The second optical scanning lens system is configured to gather light of the laser light beam from the first optical scanning lens system in an approximately linear state extended in a main scanning direction. The rotary polygon mirror has a predetermined number N of deflective reflection surfaces and is configured to receive and deflect the laser light beam gathered in the approximately linear state. The third optical scanning lens system is configured to gather the deflected laser light beam from the rotary polygon mirror to form a beam spot on an imaging surface. In this novel optical scanning apparatus, the predetermined number M satisfies a condition:
3xc3x97Rp/Rmaxxe2x89xa7Mxe2x89xa7Rp/Rmax,
wherein Rp and Rmax are defined as:
Rpxe2x89xa1(Dpi/25.4)xc3x97(260xc3x97Ppm)/N,
Rmaxxe2x89xa1(3.8xc3x97106)xc3x97√{N1.6/(A4xc3x97t)}, respectively,
wherein M is greater than two, Rp is a revolution number (rpm) of the rotary polygon mirror in a single beam mode, Rmax is a maximum revolution number (rpm) of the rotary polygon mirror, Dpi, is a pixel density (dpi) in a sub-scanning direction, Ppm is a print speed (ppm) expressed in a number of A4-sized print pages in a landscape orientation, N is a number of the deflective reflection surfaces of the rotary polygon mirror, A is a radius (mm) of an inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror, and t is a thickness (mm) of each deflective reflection surface of the rotary polygon mirror.
A diameter xcfx89m of the beam spot in the main scanning direction, formed on the imaging surface by the third optical scanning lens system, may be a 1/e2 diameter and is equal to or smaller than 50 xcexcm, and the radius A of the inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror may satisfy a condition:
Axe2x89xa7(0.76xc3x97fmxc3x97xcex/xcfx89m)/tan(180xc2x0/N),
wherein xcex (mm) is a central wave length of the light source, fm (mm) is a focal distance of the third optical scanning lens system in the main scanning direction, and xcex8 is a half field angle of the third optical scanning lens system, including a synchronizing laser light beam.
The predetermined number M of light sources may be made of a monolithic semiconductor laser array.
The predetermined number M of light sources may be packaged in a single light source unit and light rays emitted by the light sources are synthesized to a single light beam.
The rotary polygon mirror may be driven by an air bearing motor.
Each of the diameter xcfx89m in the main scanning direction and a diameter in a sub-scanning direction with respect to the beam spot formed on the imaging surface by the third optical scanning lens system may be a 1/e2 diameter and equal to or smaller than 50 xcexcm.
The third optical scanning lens system may include at least two optical devices that include at least one surface having a non-circular-arc shape in the main scanning and sub-scanning directions.
To achieve the above-mentioned and other objects, in one example, a novel optical scanning method using an underfilled optical system includes the steps of emitting, performing, collecting, rotating, and gathering. The emitting step emits a laser light beam with a predetermined number M of light sources. The performing step performs a coupling process with a first optical scanning lens system relative to the laser light beam emitted from the predetermined number M of light sources. The collecting step collects light of the laser light beam from the first optical scanning lens system in an approximately linear state extended in a main scanning direction using a second optical scanning lens system. The rotating step rotates a predetermined number N of deflective reflection surfaces of a rotary polygon mirror to deflect the laser light beam gathered in the approximately linear state. The gathering step gathers the deflected laser light beam deflected by the deflective reflection surfaces of the rotary polygon mirror to form a beam spot on an imaging surface using a third optical scanning lens system. In this novel method, the predetermined number M satisfies a condition:
3xc3x97Rp/Rmaxxe2x89xa7Mxe2x89xa7Rp/Rmax,
wherein Rp and Rmax are defined as:
xe2x80x83Rpxe2x89xa1(Dpi/25.4)xc3x97(260xc3x97Ppm)/N,
Rmaxxe2x89xa1(5.4xc3x97106)xc3x97√{N1.6/(A4xc3x97t)}, respectively,
wherein M is greater than two, Rp is a revolution number (rpm) of the rotary polygon mirror in a single beam mode, Rmax is a maximum revolution number (rpm) of the rotary polygon mirror, Dpi is a pixel density (dpi) in a sub-scanning direction, Ppm is a print speed (ppm) expressed in a number of A4-sized print pages in a landscape orientation, N is a number of the deflective reflection surfaces of the rotary polygon mirror, A is a radius (mm) of an inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror, and t is a thickness (mm) of each deflective reflection surface of the rotary polygon mirror.
A diameter xcfx89m of the beam spot in the main scanning direction, formed on the imaging surface by the third optical scanning lens system, may be a 1/e2 diameter and is equal to or smaller than 50 xcexcm, and the radius A of the inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror may satisfy a condition:
Axe2x89xa7(0.76xc3x97fmxc3x97xcex/xcfx89m)/tan {(180xc2x0/N)xe2x88x92(xcex8/2)},
wherein xcex (mm) is a central wave length of the light source, fm (mm) is a focal distance of the third optical scanning lens system in the main scanning direction, and xcex8 is a half field angle of the third optical scanning lens system, including a synchronizing laser light beam.
In another example, a novel optical scanning method using an overfilled optical system includes the steps of emitting, performing, collecting, rotating, and gathering. The emitting step emits a laser light beam with a predetermined number M of light sources. The performing step performs a coupling process using a first optical scanning lens system relative to the laser light beam emitted from the predetermined number M of light sources. The collecting step gathers light of the laser light beam from the first optical scanning lens system in an approximately linear state extended in a main scanning direction using a second optical scanning lens system. The rotating step rotates a predetermined number N of deflective reflection surfaces of a rotary polygon mirror to deflect the laser light beam gathered in the approximately linear state. The gathering step gathers the laser light beam deflected by the rotary polygon mirror to form a beam spot on an imaging surface using a third optical scanning lens system. In this novel method, the predetermined number M satisfies a condition:
3xc3x97Rp/Rmaxxe2x89xa7Mxe2x89xa7Rp/Rmax,
wherein Rp and Rmax are defined as:
Rpxe2x89xa1(Dpi/25.4)xc3x97(260xc3x97Ppm)/N,
Rmaxxe2x89xa1(3.8xc3x97106)xc3x97√{N1.6/(A4xc3x97t)}, respectively,
wherein M is greater than two, Rp is a revolution number (rpm) of the rotary polygon mirror in a single beam mode, Rmax is a maximum revolution number (rpm) of the rotary polygon mirror, Dpi is a pixel density (dpi) in a sub-scanning direction, Ppm is a print speed (ppm) expressed in a number of A4-sized print pages in a landscape orientation, N is a number of the deflective reflection surfaces of the rotary polygon mirror, A is a radius (mm) of an inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror, and t is a thickness (mm) of each deflective reflection surface of the rotary polygon mirror.
A diameter xcfx89m of the beam spot in the main scanning direction, formed on the imaging surface by the third optical scanning lens system, may be a 1/e2 diameter and is equal to or smaller than 50 xcexcm, and the radius A of the inscribed circle with respect to the deflective reflection surfaces of the rotary polygon mirror may satisfy a condition:
A((0.76xc3x97fmxc3x97(/(m)/tan(180/N),
wherein xcex (mm) is a central wave length of the light source, fm (mm) is a focal distance of the third optical scanning lens system in the main scanning direction, and xcex8 is a half field angle of the third optical scanning lens system, including a synchronizing laser light beam.
The predetermined number M of light sources may be made of a monolithic semiconductor laser array.
The predetermined number M of light sources may be packaged in a single light source unit and light rays emitted by the light sources are synthesized to a single light beam.
The deflective reflection surfaces may be driven by an air bearing motor.
Each of the diameter xcfx89m in the main scanning direction and a diameter in the sub-scanning direction with respect to the beam spot formed on the imaging surface by the third optical scanning lens system may be a 1/e2 diameter and is equal to or smaller than 50 xcexcm.
The third optical scanning lens system may include at least two optical devices that include at least one surface having a non-circular-arc shape in the main scanning and sub-scanning directions.
In another example, a novel image forming apparatus that prints at a speed of 50 ppm or higher expressed in a number of A4-sized print pages in a landscape orientation and at a pixel density of 1200 dpi includes any one of the above-mentioned optical scanning apparatuses.